The present invention pertains to a filter for the purification of drinking water and, more particularly, to a filter particularly adapted for treating a domestic drinking water supply to remove a broad range of potentially harmful organisms, including bacteria, viruses and protozoan cysts.
Waterborne diseases are known to be caused by a wide variety of organisms, including bacteria, viruses and protozoa. In the treatment of drinking water supplies, these organisms are typically removed by a combination of disinfection and filtration. Thus, a broad range of disinfecting chemicals and filter media are known and used. Increasing concern over the contamination of certain surface and ground water supplies used as sources of drinking water have lead to a careful examination of conventional water treatment mechanisms to assure that pathogenic organisms of all types are effectively removed.
For some time, it has been suspected that disease-transmitting viruses may, in some instances, not be effectively removed in conventional water treatment systems. This may be attributable to either their resistance to disinfectants typically found effective to kill bacteria and other organisms or their small size which renders virtually all filter media ineffective to retain and remove viruses. In addition, waterborne diseases carried by bacteria and protozoa also remain a problem, particularly where polluted surface waters must be utilized and treated to supply drinking water.
A wide variety of relatively small filter units have been developed for home use or the like in treating drinking water which may have already been treated in a municipal treatment system. Such filters may be of a size adequate to treat all incoming water to a home or, more typically, a smaller size which may be attached directly to a faucet to provide an immediate source of supplementally treated drinking water. These small filters have used a variety of media to provide supplemental microbiocidal activity in an attempt to eliminate any kind of pathogenic organisms which may have survived municipal treatment. These media include activated carbon, semipermeable membranes, and a variety of disinfectants. In addition, these or other types of filter media are sometimes also utilized in small filter units to provide supplemental removal of other contaminants, such as heavy metals, and other dissolved and suspended gases and particulates.
The microbiocidal activity of silver and certain silver compounds is well known and these materials are often mixed with another filter media, such as activated carbon, to provide a combination of biocidal activity and mechanical filtration to remove a wide range of organisms. However, at the safe upper levels of silver typically allowed in drinking water filters, the microbiocidal activity is typically less than complete. Certain types of small pore activated carbon filters can effectively retain larger microorganisms such as protozoan cysts and some bacteria, but are totally ineffective against the very much smaller viruses. Even semipermeable membrane filters which may have the capability of retaining organisms and other particles as small as 0.2 micron, are also ineffective in retaining viruses.
A wide variety of disinfecting resins are also well known and widely used in filters for the treatment of drinking water. Those resins applied particularly to kill microbiological pathogens in water typically utilize halogenated anion exchange resins. One particularly effective disinfecting resin is an iodinated polyvinylpyridine resin of a type disclosed in U.S. Pat. No. 4,594,392.
U.S. Pat. No. 4,769,143 discloses a multimedia filter for purifying drinking water which includes one or more disinfecting media described as effective to kill bacteria in the drinking water supply. The bacteriacidal filter media disclosed in the foregoing patent include a mixed bed of crystalline iodine and an anion exchange resin, and a silver-impregnated activated carbon element. In addition, a semipermeable membrane element disposed between the resin and carbon media provides a supplemental mechanical filtration of bacteria not killed in the iodine/resin bed. However, the microbiocidal effectiveness of a typical halogenated disinfecting resin depends largely on the contact or residence time of the water to be treated in the resin media. Although contact time or residence may be simply a function of the size of the resin bed, practical considerations limit such size and, in the case of small filter units intended for household use and attachment to a single faucet, the volume available for the disinfecting resin is quite limited. Therefore, residence time of the water in the filter necessary to provide effective microbiocidal activity requires a limitation on the flow rate of the water through the filter unit. Obviously, if the filter unit is used in a water supply with wide fluctuations in supply pressure, higher system pressures may reduce the residence time of the water in the disinfecting media to a point where large numbers of organisms are unaffected and pass through the filter. Backup media, such as membranes or bacteriostatic carbon elements may help, but do not assure removal or kill of pathogenic organisms passing through the disinfecting resin bed. It is also known that decreasing supply water temperature decreases the disinfecting activity of the typical halogenated resin, such that effective microbiocidal activity may be lost with decreasing supply water temperature.
There is a need, therefore, for a small portable filter for the treatment of drinking water which will effectively remove a wide range of microorganic pathogens, including bacteria, viruses and protozoa or protozoan cysts. Such a filter unit should be effective over a wide range of supply water pressures. The use of a halogenated disinfecting resin should also include appropriate means for eliminating residual free halogens which may be flushed from the resin. The filter unit should also preferably be constructed to effectively treat water over an anticipated range of supply water temperature and of varying pH.